It has been reported that over 4 million concussions occur in sport and recreational activities. Studies of professional football players in the United States National Football League (NFL) indicate that approximately 150 players or 1 player in every 2-3 games are diagnosed with a mild traumatic brain injury (MTBI) also referred to as a mild concussion. A study of over 1,000 former NFL players found that 60% had sustained at least one concussion and 26% had sustained three or more concussions in their career. Another study of retired NFL players suggested that 20% of players that sustained a concussion subsequently suffered from depression, which is three times the rate of depression in players who have not sustained a concussion. The risks of long-term cognitive deficit and cumulative effects of multiple concussions highlight the need for improvements in helmet design.
While various types of protective headgear have been developed to reduce injuries to the brain, skull, and neck resulting from head impacts using a hard outer shell in combination with internal padding made of an energy-absorbing material, conventional helmet designs are generally based on reducing injuries associated with linear (translational) acceleration due to impacts. Conventional designs having internal padding secured to the outer shell by chemical fasteners, such as adhesives, or by mechanical fasteners, such as hook and loop closures, or similar structures, for example, may not adequately protect users from various types of traumatic brain injury (TBI), which may include diffuse brain injury (DBI) that forms a broad spectrum of injuries ranging from mild concussion without loss of consciousness, to classical cerebral concussion with transient disturbance of consciousness, to diffuse axonal injury (DAI) with prolonged loss of consciousness of varying duration and subdural hematoma. TBI may result from impacts with another player, an object, or the ground, for example. While these types of injuries have been recognized in the prior art, there is disagreement with respect to the injury mechanism and therefore with respect to methods for mitigating these types of injuries using an appropriate helmet. Likewise, prior art helmet design and testing based primarily on linear (translational) acceleration as a predictor of injury resulting from an impact may be less reliable than more recently developed indicators.